Specific Protein Attachment to Artificial Membranes via Coordination to Lipid-Bound Copper(II)

A versatile and convenient method for targeting proteins to lipid assemblies using metal ion coordination is described. Mixed lipid bilayers and Langmuir monolayers containing a metal-chelating lipid and divalent copper ions are shown to bind protein via surface-accessible histidine residues. Cu²⁺ chelated by iminodiacetate (IDA) in the headgroup serves as an affinity ligand to target the protein to the interface. The compact, uncharged Cu²⁺‒IDA headgroup can be incorporated into lipid assemblies without disrupting the lipid packing. Surface pressure‒area isotherms of DSPC monolayers containing 5 mol % of IDA‒lipid show that Cu²⁺ enhances the rate and extent of myoglobin association with the interface. Myoglobin binds to small unilamellar vesicles containing 2% Cu²⁺‒IDA lipid (48% DSPC and 50% cholesterol) at least an order of magnitude more tightly than to vesicles without metal or loaded with Ca²⁺. The Cu²⁺‒IDA lipid more than doubles the amount of protein targeted to the interface. Cu²⁺ ESR parameters g_∥ and A_∥, measured for liposomes with native and DEPC-modified myoglobin, support coordination of surface histidine side chains to Cu²⁺ as the binding interaction.